In recent years, various display substrates using plastic film or sheet as a substrate have been proposed, and the substrate is required to have barrier properties to water vapor and oxygen. For giving such barrier properties to the substrate, the substrate is often subjected to coating with a transparent SiOx film, and a productive coating means therefor is desired. As techniques for coating the substrate with the SiOx film in the process of carrying the substrate from a roll to a roll, for example, physical vapor deposition (PVD) method such as vacuum evaporation or sputtering, and plasma CVD (plasma enhanced-chemical vapor deposition) method are known.
The vacuum evaporation method is extensively used to perform film formation mainly to food packaging films as a productive process, but the resulting barrier performance does not satisfy a level requested as display substrate, with water vapor transmission and oxygen transmission of about 1 g/m2·day and about 1 cc/m2·atm·day. On the other hand, a denser film can be formed by the sputtering method. For example, barrier performances of not more than 0.02 g/m2·day and 0.02 cc/m2·atm·day that are detection limits of MOCON method can be attained by forming a SiOx or SiON film of 50-100 nm on a substrate in good surface condition. However, the deposition rate is too low to ensure sufficient productivity. Further, since a film formed by the PVD method is inorganic and brittle, the film, when formed in a thickness exceeding 100 nm, is easily subject to film defects or peeling, resulting from an internal stress of the film or a difference in thermal expansion coefficient between the film and the substrate, and further resulting from the failure of film to follow deformation of the base film.
In contrast, the plasma CVD method is inferior to the vacuum deposition method, but has superiority of one order of magnitude or more in terms of deposition rate to the sputtering method, and thus has a possibility that a high-barrier film can be formed. This method further has a feature that a film as thick as several hundreds nm to several μm which cannot be attained by the PVD method can be formed on the base film, since a film formed thereby has a certain level of flexibility. Therefore, the plasma CVD method is expected as a new film forming process utilizing these features.
Various types of film forming apparatuses by plasma CVD are conventionally known. As an apparatus adapted to perform film formation while winding a film around a deposition roll, for example, an apparatus including a pair of deposition rolls for winding and carrying a film that is a deposition object is described in Japanese translation of PCT International Application No. 2005-504880 (Patent Literature 1), in which a magnetic field is formed to extend between the rolls, and the pair of deposition rolls is connected to a high frequency power source so that the two deposition rolls have the same polarity, and a high frequency power of several tens to several hundreds kHz is simultaneously supplied thereto to cause penning discharge in a space (discharge area) between the rolls to confine plasma, and oxygen and a raw material gas such as HMDSO are supplied to the space between the rolls to consequently perform film formation simultaneously to the film on the deposition rolls at both sides of the discharge area.
Further, a plasma CVD apparatus is described in Japanese Patent No. 2587507 (Patent Literature 2), and the apparatus comprises a pair of deposition rolls (metal drums) disposed oppositely to each other within a vacuum chamber, an AC power source having one electrode connected to one of the deposition rolls and the other electrode connected to the other deposition roll, a discharge chamber disposed in a space between the deposition rolls with the faces opposed to the deposition rolls being opened, and a monomer (raw material) gas supply means connected to the discharge chamber. According to Patent Literature 2, plasma can be generated within the discharge chamber to perform film formation to a film on the deposition rolls since the inside of the discharge chamber is reduced in vacuum degree by supply of a monomer gas, compared to the outside, and contamination of the discharge electrodes can be prevented, since the surfaces of the deposition rolls constituting discharge electrodes are covered with the film carried thereon.
However, in the film forming apparatus of Patent Literature 1, because the other electrode of the power source for discharge must be connected to an annular electrode (counter electrode) provided substantially in an equal distance from the center of the space between the deposition rolls, plasma is generated also at the periphery of the counter electrode, and it is difficult to perfectly suppress film deposition in this periphery. Further, change in discharge associated with the film deposition to the counter electrode and flaking which are likely to develop film defects, are caused during long-time operation.
On the other hand, in the film forming apparatus of Patent Literature 2, although the discharge chamber must be formed in the space between the deposition rolls, film deposition occurs on the wall of the discharge chamber, and flaking occurring from this portion is likely to develop film defects. In addition, for making the inside of the discharge chamber lower in vacuum degree (or higher pressure) than the other part within the vacuum chamber, the flow of the gas must be suppressed by extremely minimizing the gap between the discharge chamber and the deposition rolls. However, since the film deposition occurs also in the vicinity of this gap, the gas confining effect of the discharge chamber is changed, impairing the stability of deposition, and the stability of film quality is consequently reduced.
[Patent Literature 1] Japanese translation of PCT International Application No. 2005-504880
[Patent Literature 2] Japanese Patent No. 2587507